Substrate with low secondary emissions

ABSTRACT

The present invention is directed to a method and apparatus for producing a highly-textured surface on a copper substrate with only extremely small amounts of texture-inducing seeding or masking material. The texture-inducing seeding material is delivered to the copper substrate electrically switching the seeding material in and out of a circuit loop.

ORIGIN OF THE INVENTION

The invention described herein was made by employees of the UnitedStates Government and may be manufactured and used for the Governmentfor government purposes without the payment of any royalties thereon ortherefore.

This is a divisional of application Ser. No. 08/331,392 filed on Oct.26, 1994 now abandoned.

FIELD OF THE INVENTION

The present invention is directed to a method and apparatus forproducing a highly-textured surface on a copper substrate with onlyextremely small amounts of texture-inducing seeding or masking material.The purpose of the invention is to produce a surface with secondaryelectron emission properties sharply lower than those of untreatedcopper.

The present invention is particularly useful on surfaces of multistageddepressed collector electrodes of vacuum-tube amplifiers such astraveling-wave tubes or klystrons. These devices, which are widely usedin space communications, aircraft, and terrestrial applications, aredescribed in U.S. Pat. No. 3,702,951 to H. Kosmahl. Some of thesignificant factors involved in maximizing the efficiency of thesedevices include the use of electrode materials that have low secondaryelectron emission properties. This is discussed in U.S. Pat. No.4,349,424 to Sovey et al.

Specifically, these low secondary electron emission properties recoverthe maximum kinetic energy from the spent electron beam after it haspassed through the device's interaction section and entered thecollector. To have high collector efficiency, the electrodes must havelow secondary electron emission characteristics so that the impingingelectrons are not excessively reflected or re-emitted from the surfaces.Textured copper surfaces have been found to be useful for this purpose.Untreated copper surfaces have relatively high secondary electronemission levels.

Prior art methods for achieving copper surfaces with reduced secondaryelectron emission properties included coating the surfaces withcompounds such as titanium carbide or roughening the surface by variousmeans such as particle blasting. However, these prior art methodsproduce only modest reductions in secondary electron emissionproperties. Specifically, the coating approach results in acoating/substrate interface which becomes subject to delaminationfailure with vibration or repeated thermal cycling due to the differencein thermal expansion characteristics between the coating and thesubstrate. The particle-blasting approach is generally even lesseffective and poses the potential problem of leaving residual andloosely-attached particles of the abrasive material on the substrate,which may cause damage by separating from the substrate during operationand migrating to other components.

Additional methods such as the direct and continuous simultaneousion-bombardment of stainless steel, tantalum, or molybdenum targets andcopper substrates in a low-pressure environment also produce ahighly-textured copper surface which displays low secondary electronemission properties. While the secondary electron emission properties ofthe copper substrate are sharply reduced by the use of this method, theresulting surface generally is burdened with unacceptably large amountsof residual target material which poses the potential problem ofseparating from the substrate in operation by virtue of largedifferences in thermal expansion properties with the copper substrate.

It is therefore an object of the invention to produce an ion-texturedcopper substrate with very low secondary electron emission propertieswith only acceptably small (essentially trace) amounts of residualtarget material which pose no potential problem of separation from thesubstrate by virtue of thermal expansion differences.

It is a further object of the present invention to use electrical meansto turn the bombardment of the substrate with the target material offand on to control and minimize the deposition or arrival rate of thetarget material on the substrate.

DESCRIPTION OF RELATED ART

U.S. Pat. No. 3,604,970 to Culbertson discloses an electrode which hasbeen coated with a non-emissive coating produced through ionimplantation. U.S. Pat. No. 4,349,424 to Sovey relates to a sputteringprocess which is intended to reduce secondary emissions. U.S. Pat. No.4,417,175 to Curren discloses a sputtering process which reducessecondary emissions in which a resurfaced graphite plate is used. U.S.Pat. No. 4,607,193 to Curren discloses a triode sputtering process toapply a textured carbon coating onto a copper substrate. U.S. Pat. No.4,693,760 to Sioshansi relates to a coating process in which ionsputtering of a titanium surface is intentionally carried out to producea surface with a high porosity. U.S. Pat. No. 5,245,248 to Chanillustrates the geometry of an emitter.

SUMMARY OF THE INVENTION

The present invention discloses a method of developing asecondary-electron-suppressing highly textured copper surface withsmall, acceptable amounts of residual molybdenum texture-inducingmasking material. This is accomplished by controlling the arrival rateof a molybdenum target/seeding material at the copper surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, advantages, and novel features of the invention will bemore fully apparent from the following detailed description when read inconnection with the accompanying drawings in which:

FIG. 1. displays a schematic diagram of the ion-texturing apparatus;

FIG. 2. displays a scanning electron microscope photomicrograph of anion-textured oxygen-free, high-conductivity (OFHC) copper surface;

FIG. 3. displays a graph of the true secondary electron emission ratioas a function of primary electron energy; and

FIG. 4. displays a graph of the reflected primary electron yield indexas a function of primary electron energy.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 displays a schematic diagram of the facility developed toaccomplish the ion-texturing of copper as shown in FIG. 2. In FIG. 1 thetextured copper surface is formed by means of a triode sputteringprocess, in which a copper substrate is continuously exposed to anaccelerated argon plasma beam in a low-pressure environment while amolybdenum target/seeding element is simultaneously but intermittentlyexposed to the same plasma beam.

FIG. 1 displays a schematic diagram of the ion-texturing apparatuscontained within a vacuum chamber 100. The vacuum chamber 100 contains aplasma chamber 110, along with an annular molybdenum target 120 and acopper substrate 130 mounted on a support structure 140, all positionedas shown.

Prior to starting the texturing process, the vacuum chamber 100 isevacuated to and maintained at a pressure of from 1×10⁻⁴ to 4×10⁻⁴ torr(millimeters of mercury) by means of a pumping system 150. When thereduced-pressure condition within the vacuum chamber 100 is reached,argon gas 160 is ducted into the plasma chamber 110 through aresistance-heated hollow cathode 170, the heater of which is served by acathode heater power supply 180. At the same time, an anode power supply190 establishes a potential difference between the cathode 170 and theplasma chamber anode 200, and an arc rf starter 210 served by a starterpower supply 220 is momentarily activated to establish a quiescentplasma 230 within the plasma chamber 110.

The texturing process begins when a potential difference is establishedbetween the cathode 170 and both the molybdenum target 120 and coppersubstrate 130 by means of a high-voltage power supply 240. Thispotential difference causes an ion beam 250 to be directed through arestricting electrically-floating orifice plate 260 and simultaneouslyimpinge the molybdenum target 120 and copper substrate 130. The ion beam250 removes small quantities of material from the molybdenum target 120and deposits it in a discontinuous but uniform "mask" over the surfaceof the copper substrate 130. Formation of the textured surface on thecopper substrate 130 apparently comes about as the result of thedifferences in sputtering properties between molybdenum and copper, withthe copper being sputtered away by the ion beam far more rapidly thanthe molybdenum dispersed masking to form densely-packed, uniform, andsharply-defined conical copper spires. During the texturing process, theion beam 250 continuously impinges the copper substrate 130 as a resultof the potential difference between the cathode 170 and the coppersubstrate 130. A high-speed switch module 270 controls the potentialdifference between the copper substrate 130 and the molybdenum target120. When the switch module 270 provides for the molybdenum target 120and the copper substrate 130 to be at the same potential relative to thecathode 170, the ion beam 250 deposits material from the molybdenumtarget 120 to the copper substrate 130. When the switch module 270electrically disconnects the molybdenum target 120 from the coppersubstrate 130 (except for a small sustaining current), the deposition ofmaterial from the molybdenum target 120 to the copper substrate 130 issharply reduced or eliminated. By regulating the switch module 270, thearrival rate of material from the molybdenum target 120 to the coppersubstrate 130 is controlled. An oscillator switch control 290 regulatesthe switch module 270 and establishes the switching frequency as well asthe duty cycle, or that portion of each switching cycle during which themolybdenum target 120 and copper substrate 130 are electricallyconnected. Hence, a switching frequency of 20 Hz with a duty cycle of 25percent indicates the molybdenum target 120 and the copper substrate 130are electrically connected 25 percent of the time (0.0125 second) duringeach of twenty switching cycles (0.05 second each) which occur duringone second. The output signals of the oscillator switch control 290 andswitch module 270 are monitored with the use of a duty cycle monitor280. During processing, a pre-selected current level is establishedbetween the cathode 170 and the copper substrate 130. During the periodwhen the molybdenum target 120 is electrically disconnected from thecopper substrate 130 by the switch module 270, the entire pre-selectedcurrent (except for a small sustaining current) passes through thecopper substrate 130 and its support structure 140. During the periodwhen the switch module 270 electrically connects the molybdenum target120 and the copper substrate 130, the pre-selected current flow isdivided between the molybdenum target 120 and the copper substrate 130.

In a typical case, where the copper substrate 130 is circular in shape,the molybdenum target 120 is annular, approximately the same diameter asthe copper substrate 130 with the target surface facing the ion beam 250sloping radially upward at an angle of approximately 45 degrees, andspaced approximately one-half the diameter of the copper substrate 130from the copper substrate 130. This configuration has been shown toprovide for uniform development of the textured surface of the coppersubstrate 130.

The operating conditions which were used to produce the textured coppersurface displayed in FIG. 2 and whose secondary electron emissioncharacteristics are displayed in FIGS. 3 and 4 are as follows:

Vacuum chamber pressure: 1×10⁻⁴ to 4×10⁻⁴ torr (millimeters of mercury)

Ionizing gas: argon

Accelerating potential difference: 1500 volts dc

Substrate current density (approx.): 5 mA/cm²

Texturing period: 1 hour

Switching frequency: 20 Hz

Duty cycle: 25 percent

FIG. 2 is a scanning electron microscope photomicrograph of anion-textured copper surface produced by the process described in thisdisclosure, taken at an angle of 30 degrees from normal to the surface.The morphology is characterized by a dense uniform array of pointedspires normal to the surface with average feature height and separationof approximately 10 and 5 micrometers, respectively. Examinations ofthis surface by Auger and energy-dispersive X-ray analyses indicate thepresence of only extremely small amounts of the texture-inducing seedingor masking material,molybdenum.

The textured copper surface characteristics and consequently thesecondary electron emission characteristics may be altered from those ofthe surface described in this disclosure by varying the operatingconditions within reasonable limits. It will be appreciated that othergases in addition to argon can be used to form the bombarding ions inthe ion beam. For example, xenon may also be used.

The secondary electron emission characteristics for normal beamimpingement and 30 and 60 degrees from normal beam impingement for arepresentative primary electron beam energy range for the surfaceproduced in accordance with the present invention are shown in FIGS. 3and 4. The true secondary electron emission ratio, which is the ratio oflow energy emitted electrons to impinging electrons for the conditionsdescribed, is shown by the curves in FIG. 3. The reflected primaryelectron yield indexed to that of a sooted control surface at the sameprimary beam energy and beam impingement angle, is shown by the curvesin FIG. 4. The measurements were made using a method described in"Secondary Electron Emission Characteristics of Molybdenum-MaskedIon-Textured OFHC Copper", NASA TP-2967, 1990, by Curren, A. N., Jensen,K. A., and Roman, R. F.

For comparison, characteristics curves for untreated copper are alsoshown in FIGS. 3 and 4, respectively. The sharply lower emission levelfor the surface of the present invention relative to those of theuntreated copper are clearly evident. The true secondary electronemission ratio increases with beam impingement angle over the entireenergy range for both the untreated and ion-textured copper surfaces,with the values for ion-textured copper ranging from 40 to 60 percentlower than those for untreated copper, as shown in FIG. 3. FIG. 4indicates the reflected primary electron yield index for the texturedcopper is significantly lower than that for untreated copper for allbeam impingement angles across the entire energy range, ranging from aminimum of about 30 percent at 60 degrees beam impingement angle toabout 70 percent at 0 degrees beam impingement angle.

Most of the electrons in a primary electron beam which impinges thetextured copper surface strike the conical sloping walls of the spiresor at the base of the spires. Secondary electrons which are then emittedfrom these regions are repeatedly intercepted with repeated partialretention by nearby adjacent spire walls. This greatly reduces netelectron emission from the textured surface. The highly textured surfacetherefore is the principal factor in producing the low secondaryelectron emission characteristics exhibited by the surface of thepresent invention relative to those of untreated copper.

While the preferred embodiment of the invention is disclosed anddescribed it will be apparent that various modifications may be madewithout departing from the spirit of the invention or the scope of thesubjoined claims.

What is claimed:
 1. A copper substrate, comprising:at least one surface,said surface being circular in shape; a target material located on saidsurface, wherein said target material is molybdenum; a plurality ofspires formed within said surface, wherein said spires texturize thesurface to form a textured surface within said substrate, wherein saidspires in said surface of said substrate are created by removingparticles of said substrate while said substrate is masked by adiscontinuous layer of said molybdenum target material, and wherein eachof said plurality of spires is spaced about 5 micrometers apart and hasa height of about ten micrometers, and wherein said target material isdelivered to said substrate by electrically switching the targetmaterial in and out of a circuit loop.
 2. A substrate, as described inclaim 1, wherein said spires exhibit a true secondary electron emissionratio of about 0.34 to about 0.48 for a primary electron energy level ofabout 200 to 2000 electron volts and an electron beam impingement angleof 0 degrees.
 3. A substrate, as described in claim 1 wherein saidspires exhibit a true secondary electron emission ratio of about 0.39 toabout 0.5 for a primary electron energy level of about 200 to 2000electron volts and an electron beam impingement angle of 30 degrees. 4.A substrate, as described in claim 1 wherein said spires exhibit a truesecondary electron emission ratio of about 0.48 to about 0.6 for aprimary electron energy level of about 200 to 2000 electron volts and anelectron beam impingement angle of 60 degrees.
 5. A substrate, asdescribed in claim 1 wherein said spires exhibit a reflected primaryelectron yield index of about 2.3 to about 12.2 for a primary electronenergy level of about 200 to 2000 electron volts and an electron beamimpingement angle of about 0 degrees.
 6. A substrate, as described inclaim 1 wherein said spires exhibit a reflected primary electron yieldindex of about 2.5 to about 14.0 for a primary electron energy level ofabout 200 to 2000 electron volts and an electron beam impingement angleof 30 degrees.
 7. A substrate, as described in claim 1 wherein saidspires exhibit a reflected primary electron yield index of about 3 toabout 22.0 for a primary electron energy level of about 200 to 2000electron volts and an electron beam impingement angle of 60 degrees.